According to a rotor magnetic strip of a motor with different magnetizing shapes and a waveform of a back electromotive force generated from a coil of the motor in a magnetic field, some coil currents of the motors are suitable for square waves, and some are suitable for sine waves. Moreover, some of the motors need to have extra switching controls to cooperate with inductance characteristics of the coils.
Generally, a conduction of a driving force of a motor driven by sine wave voltage is smoother, and lower vibration noise. In practice, for the motor with coil currents suitable for sine waves, circuit design engineers usually adjust the waveform of a three-phase driving voltage provided to the motor from a sine wave to a sine wave with a double-peaked waveform (i.e. discontinuous PWM (DPWM) waveform).
However, due to the influences of some other factors, such as non-ideal characteristics of circuit components, even if the waveform of the three-phase driving voltage of the motor is adjusted to the double-peaked sine wave from the sine wave, the coil current of the motor still cannot match the shape of the ideal waveform of the sine wave. The aforementioned situation is especially obvious when the motor operates at low speed.